Megrez , also called Delta Ursae Majoris (δ Ursae Majoris, abbreviated Delta UMa, δ UMa), is a

star A star is a luminous spheroid of plasma (physics), plasma held together by Self-gravitation, self-gravity. The List of nearest stars and brown dwarfs, nearest star to Earth is the Sun. Many other stars are visible to the naked eye at night sk ...

in the northern

constellation A constellation is an area on the celestial sphere in which a group of visible stars forms Asterism (astronomy), a perceived pattern or outline, typically representing an animal, mythological subject, or inanimate object. The first constellati ...

Ursa Major Ursa Major, also known as the Great Bear, is a constellation in the Northern Sky, whose associated mythology likely dates back into prehistory. Its Latin name means "greater (or larger) bear", referring to and contrasting it with nearby Ursa M ...

. With an

apparent magnitude Apparent magnitude () is a measure of the Irradiance, brightness of a star, astronomical object or other celestial objects like artificial satellites. Its value depends on its intrinsic luminosity, its distance, and any extinction (astronomy), ...

of +3.3, it is the dimmest of the seven stars in the

Big Dipper The Big Dipper (American English, US, Canadian English, Canada) or the Plough (British English, UK, Hiberno-English, Ireland) is an asterism (astronomy), asterism consisting of seven bright stars of the constellation Ursa Major; six of them ar ...

asterism.

Parallax Parallax is a displacement or difference in the apparent position of an object viewed along two different sightline, lines of sight and is measured by the angle or half-angle of inclination between those two lines. Due to perspective (graphica ...

measurements yield a distance estimate of from the

Sun The Sun is the star at the centre of the Solar System. It is a massive, nearly perfect sphere of hot plasma, heated to incandescence by nuclear fusion reactions in its core, radiating the energy from its surface mainly as visible light a ...

Stellar properties

Megrez has two times more mass than the Sun and is about 23 times more luminous. It has a

stellar classification In astronomy, stellar classification is the classification of stars based on their stellar spectrum, spectral characteristics. Electromagnetic radiation from the star is analyzed by splitting it with a Prism (optics), prism or diffraction gratin ...

of A3 V, which means it is an

A-type main sequence star An A-type main-sequence star (A) or A dwarf star is a main-sequence (hydrogen burning) star of spectral type A and luminosity class (five). These stars have spectra defined by strong hydrogen Balmer absorption lines. They measure between 1.7 ...

that is generating energy at its core through the

nuclear fusion Nuclear fusion is a nuclear reaction, reaction in which two or more atomic nuclei combine to form a larger nuclei, nuclei/neutrons, neutron by-products. The difference in mass between the reactants and products is manifested as either the rele ...

of hydrogen. It rotates rapidly, taking 3.1 hours to complete a rotation across its

equator The equator is the circle of latitude that divides Earth into the Northern Hemisphere, Northern and Southern Hemisphere, Southern Hemispheres of Earth, hemispheres. It is an imaginary line located at 0 degrees latitude, about in circumferen ...

, causing Megrez to have an oblate shape and hotter temperatures at the poles due to

gravity darkening Gravity darkening, also referred to as gravity brightening, is an astronomical phenomenon where the poles of a star are brighter than the equator, due to rapid rotation and oblate shape. When a star is oblate, it has a larger radius at its equat ...

. This star has an excess emission of infrared radiation, indicating the presence of circumstellar matter. This forms a

debris disk A debris disk (American English), or debris disc ( Commonwealth English), is a circumstellar disk of dust and debris in orbit around a star. Sometimes these disks contain prominent rings, as seen in the image of Fomalhaut on the right. Debris ...

around an orbital radius of 16

astronomical unit The astronomical unit (symbol: au or AU) is a unit of length defined to be exactly equal to . Historically, the astronomical unit was conceived as the average Earth-Sun distance (the average of Earth's aphelion and perihelion), before its m ...

s from the star. This radius is unusually small for the estimated age of the disk, which may be explained by drag from the

Poynting–Robertson effect The Poynting–Robertson effect, also known as Poynting–Robertson drag, named after John Henry Poynting and Howard P. Robertson, is a process by which solar radiation causes a dust grain orbiting a star to lose angular momentum relative to its or ...

causing the dust to spiral inward. It has two faint companions, a 10th magnitude star and an 11th magnitude star, both at an

angular separation Angular distance or angular separation is the measure of the angle between the orientation of two straight lines, rays, or vectors in three-dimensional space, or the central angle subtended by the radii through two points on a sphere. When t ...

of two

arcminute A minute of arc, arcminute (abbreviated as arcmin), arc minute, or minute arc, denoted by the symbol , is a unit of angular measurement equal to of a degree. Since one degree is of a turn, or complete rotation, one arcminute is of a tu ...

s from the primary. Delta Ursae Majoris is a member of the

Ursa Major moving group The Ursa Major Moving Group, also known as Collinder 285 and the Ursa Major association, is the closest Stellar kinematics#Moving groups, stellar moving group – a set of stars with common velocities in space and thought to have a common orig ...

, an

association Association may refer to: *Club (organization), an association of two or more people united by a common interest or goal *Trade association, an organization founded and funded by businesses that operate in a specific industry *Voluntary associatio ...

of stars that share a common motion through space and likely formed in the same

molecular cloud A molecular cloud—sometimes called a stellar nursery if star formation is occurring within—is a type of interstellar cloud of which the density and size permit absorption nebulae, the formation of molecules (most commonly molecular hydrogen, ...

. The space velocity components of Delta Ursae Majoris in the

galactic coordinate system The galactic coordinate system is a celestial coordinate system in spherical coordinates, with the Sun as its center, the primary direction aligned with the approximate center of the Milky Way Galaxy, and the fundamental plane (spherical coordin ...

are

, V, W The comma is a punctuation mark that appears in several variants in different languages. Some typefaces render it as a small line, slightly curved or straight, but inclined from the vertical; others give it the appearance of a miniature fille ...

= 15.35, +1.17, –11.52

Nomenclature

''δ Ursae Majoris'' ( Latinised to ''Delta Ursae Majoris'') is the star's

Bayer designation A Bayer designation is a stellar designation in which a specific star is identified by a Greek alphabet, Greek or Latin letter followed by the genitive case, genitive form of its parent constellation's Latin name. The original list of Bayer design ...

. It bore the traditional name ''Megrez'' and the historical name ''Kaffa''. ''Megrez'' comes from the ''al-maghriz'' 'the base f the bear's tail. Professor Paul Kunitzch has been unable to find any clues as to the origin of the name ''Kaffa'', which appeared in a 1951 publication, ''Atlas Coeli'' (

Skalnate Pleso Atlas of the Heavens The ''Skalnaté Pleso Atlas of the Heavens'' (''Atlas Coeli Skalnaté Pleso 1950.0'') is a set of 16 celestial charts covering the entire sky. It is named after the Skalnaté Pleso Observatory in Slovakia where it was produced. The first versi ...

) by Czech astronomer

Antonín Bečvář Antonín Bečvář (; 10 June 1901 – 10 January 1965) was a Czechoslovak astronomer. Biography He was born and died in Stará Boleslav. Among his chief achievements is the foundation of the Skalnaté Pleso Observatory and the discovery of the ...

. The

Hindu Hindus (; ; also known as Sanātanīs) are people who religiously adhere to Hinduism, also known by its endonym Sanātana Dharma. Jeffery D. Long (2007), A Vision for Hinduism, IB Tauris, , pp. 35–37 Historically, the term has also be ...

s knew this star as ''

Atri Atri or Attri is a Vedic sage, who is credited with composing numerous shlokas to Agni, Indra, and other Vedic deities of Hinduism. Atri is one of the Saptarishi (seven great Vedic sages) in the Hindu tradition, and the one most mentioned in ...

'', one of the Seven Rishis. In

Chinese Chinese may refer to: * Something related to China * Chinese people, people identified with China, through nationality, citizenship, and/or ethnicity **Han Chinese, East Asian ethnic group native to China. **'' Zhonghua minzu'', the supra-ethnic ...

, (), meaning ''

Northern Dipper The Big Dipper ( US, Canada) or the Plough ( UK, Ireland) is an asterism consisting of seven bright stars of the constellation Ursa Major; six of them are of second magnitude and one, Megrez (δ), of third magnitude. Four define a "bowl" or ...

'', refers to an asterism equivalent to the Big Dipper. Consequently, the

Chinese name Chinese may refer to: * Something related to China * Chinese people, people identified with China, through nationality, citizenship, and/or ethnicity **Han Chinese, East Asian ethnic group native to China. **'' Zhonghua minzu'', the supra-ethni ...

for Delta Ursae Majoris itself is (, ) and (, ).AEEA (Activities of Exhibition and Education in Astronomy) 天文教育資訊網 2006 年 6 月 15 日

Namesakes

USS ''Megrez'' (AK-126) was a

United States Navy The United States Navy (USN) is the naval warfare, maritime military branch, service branch of the United States Department of Defense. It is the world's most powerful navy with the largest Displacement (ship), displacement, at 4.5 millio ...

Crater class cargo ship The ''Crater''-class cargo ship were converted EC2-S-C1 type, Liberty cargo ships, constructed by the United States Maritime Commission (USMC) for use by the US Navy during World War II. The designation 'EC2-S-C1': 'EC' for Emergency Cargo, '2' ...

named after the star.

Notes

References

{{Stars of Ursa Major A-type main-sequence stars Triple star systems Ursa Major moving group Megrez Ursae Majoris, Delta Big Dipper Ursa Major Durchmusterung objects Ursae Majoris, 69 106591 059774 4660